Nitrous oxide presently has many uses. If inhaled it has anesthetic properties. Because it is an oxidizing agent, it can be introduced into an engine's induction tract causing “oxygen enrichment”, allowing for the addition of supplemental fuel, thereby increasing the engine's power output. In both of these applications it is normally beneficial to have a nitrous oxide delivery system which can maintain a desired constant nitrous oxide mass delivery rate.
A property of nitrous oxide which makes this constant mass delivery rate difficult to obtain is the relatively large change in nitrous oxide bottle pressure as a function of the temperature of the nitrous oxide in the storage bottle. Nitrous oxide at normal atmospheric temperatures and pressures is a gas. It is stored in a bottle under high pressure, this high pressure raising its boiling point, causing some of the nitrous oxide to liquefy. In the bottle, therefore, the liquid nitrous oxide is in equilibrium with its vapor, and the vapor pressure of the nitrous oxide at a particular bottle temperature establishes the bottle pressure. In other words, the bottle pressure is essentially equal to the nitrous oxide vapor pressure, this vapor pressure being a function of temperature. For instance, liquid nitrous oxide stored in a bottle establishes a bottle pressure of approximately 2.76E07 dynes/cm^2 (400 pounds/ in^2 (PSI)) at −9° C. (20° F.) nitrous oxide temperature, but rises to approximately 5.52E07 dynes/cm^2 (800 PSI) at 24° C. (75° F.).
Another consideration which makes this constant mass delivery rate difficult to obtain is the fact that as nitrous oxide leaves the storage bottle, the temperature of the nitrous oxide inside the bottle decreases due to the change of enthalpy. This temperature reduction occurs rapidly causing a rapid decrease in bottle pressure which can result in a rapid decrease in mass delivery rate.
Nitrous oxide delivery systems can be divided into two types, vapor delivery systems and liquid delivery systems, determined by whether the nitrous oxide leaves the bottle primarily as a vapor or liquid. Nitrous oxide bottles contain a discharge valve at this top and they may contain a siphon tube, a tube which extends from the bottle's valve (at the bottle's top) to the bottle's bottom. A nitrous oxide bottle without a siphon tube if oriented so the valve is elevated relative to the bottle's bottom will discharge essentially only nitrous oxide vapor (the liquid of course is heavier than the vapor and lies at the bottom of the bottle and does not exit the valve). If oriented so the valve is lower than the bottom it will discharge essentially only nitrous oxide liquid. The reverse is true for a bottle with a siphon tube, discharging principally liquid nitrous oxide if its valve is elevated but vapor if its valve is low.
Nitrous oxide used as an anesthetic, for instance, uses a vapor delivery system. Liquid nitrous oxide, when released at atmospheric pressure, boils at a temperature of −88° C. (−172° F.), and the resulting cold vapor would be uncomfortable sprayed against a patient's face. In vapor delivery systems, the boiling occurs in the bottle and the resulting vapor which is released has been warmed by the bottle and delivery system before contacting the patient. Another reason is that conventional flow regulation means used to control the delivery rate of a gas, such as a pressure regulator and discharge orifice, can be used to control the nitrous oxide mass delivery rate in a vapor delivery system.
Nitrous oxide systems used as an oxidizing agent to increase an engine's power output use a liquid delivery system, one reason being opposite to that presented above for the vapor system for anesthetic use. In this case the “patient” is the engine and cold nitrous vapor delivered to this “patient” is beneficial due to its relatively high density. Injecting extremely cold nitrous oxide vapor into the engine's induction tract displaces a relatively small amount of the air which is normally drawn into the engine. If a vapor delivery system were used, the warmer nitrous oxide vapor would displace a relatively large amount of air, reducing total engine cylinder mass of fuel and oxygen relative to that attained with a liquid system, and performance would be reduced. Therefore, in nitrous oxide systems used as an engine oxidizing agent, liquid delivery systems are used and the nitrous oxide exists as principally a liquid until just before it is injected into the engine, taking best advantage of the temperature reduction in the nitrous oxide as it vaporizes.
In a nitrous oxide liquid delivery system used to increase engine power, use of a pressure regulator and discharge orifice to control mass delivery rate is not effective. When the regulator lowered the pressure of the liquid nitrous oxide from the bottle pressure to the regulated design pressure, a portion of the nitrous oxide would change from liquid to vapor to cool the nitrous oxide to the lower boiling point at the lower pressure. The nitrous oxide existing after passing through the regulator would therefore be a mixture of liquid and vapor, the proportion of liquid to vapor depending on the relationship between the unregulated bottle pressure and the regulated design pressure. The mass density at the entrance to the discharge orifice would be a function of this liquid/vapor proportion and therefore the mass delivery rate would still depend on bottle pressure. Since a pressure regulator and orifice does not maintain the desired constant mass delivery rate in a liquid delivery system, present liquid systems used to increase engine power use a fixed orifice to control mass delivery rate, this fixed orifice normally being located in a spray nozzle.
In a liquid nitrous oxide delivery system which uses a fixed orifice to control mass flow, a change in nitrous oxide storage bottle pressure results in a change in nitrous oxide delivery rate. As the liquid nitrous oxide moves through the conduits, valves, and fittings of the delivery system to the main controlling orifice, pressure drops in the connecting lines, valves, and fittings result in some vaporization of the nitrous oxide. Therefore, as the nitrous oxide enters the controlling jet, or orifice, the nitrous oxide is a mixture of liquid nitrous oxide and nitrous oxide vapor. Normally, the nitrous oxide delivery system lines, valves, and fittings are sized to provide minimal pressure drop and consequent vaporization, and therefore the nitrous oxide present at the inlet to the limiting jet or orifice is mostly in the liquid state.
Presently, manufacturers of liquid delivery systems used to increase engine power caution their users concerning the effect of changing bottle pressure caused by changing nitrous oxide temperature, and offer several solutions to keep the nitrous oxide delivery rate relatively constant. One solution is to change the nitrous oxide jet (with fixed orifice) with changes in bottle temperature to maintain the design mass delivery rate, but this is time consuming and can require continual adjustment. Another solution is to maintain the bottle at a fixed temperature to maintain its pressure at a “design” pressure. Manufactures offer thermostatically controlled bottle heaters attempting to maintain this design pressure, but these heaters are expensive, require a relatively large power source which may not be available on smaller recreational vehicles, and the bottle's thermal time constant can present application limitations. For instance, these bottle heaters cannot respond fast enough to prevent the drop in nitrous oxide pressure which results from then nitrous oxide cooling associated with nitrous oxide delivery discussed above. Another solution is described in U.S. Pat. No. 4,494,488 To Wheatley (1985) wherein an additional bottle of high pressure nitrogen gas is used to maintain a relatively constant nitrous oxide pressure. This requires the use of an additional high pressure bottle and regulator which adds weight, cost, and complexity to the nitrous oxide delivery system.
Other applications let the nitrous oxide pressure change, resulting in a nitrous oxide delivery rate change, but adjust the supplemental fuel enrichment in order to keep the fuel/oxygen ratio correct. In U.S. Pat. No. 6,105,563 to Patrick (2000), a nitrous oxide pressure signal is used to affect the amount of supplemental fuel delivery. In another system, dynamic pressure of the exiting nitrous oxide affects the pressure in the engine's carburetor float bowls, thereby enrichening the fuel mixture in relation to the nitrous oxide bottle pressure. These systems and all similar systems which allow the nitrous oxide delivery rate to vary with bottle pressure but adjust supplemental fuel flow are systems in which the engine's power increase due to the application of nitrous oxide varies with nitrous oxide bottle pressure. At high bottle temperatures and consequently high bottle pressures, nitrous oxide flow rate is relatively high, supplemental fuel flow is relatively high, and the supplemental power is relatively high. When the bottle temperature and pressure is lower, the supplemental power is relatively lower. In other words, the engine's power when on nitrous oxide is a function of the temperature of the nitrous oxide in the bottle, and this is not normally desirable.
In other systems, which are usually less expensive systems, the nitrous oxide mass delivery rate is allowed to vary with bottle temperature and pressure, the fuel flow is not adjusted for this change, and the engine is just allowed to run leaner when the nitrous oxide pressure is higher and richer when it is lower. These systems cannot delivery optimum power because of the danger of causing engine damage when the nitrous oxide pressure is high due to too much oxygen enrichment with resulting leaner fuel mixture.